Inverter apparatus

ABSTRACT

In an inverter apparatus, an output voltage of an active filter circuit that steps up and smoothes a DC voltage is converted into an AC voltage by an inverter circuit. The active filter circuit includes a capacitor and a rectifier device connected between an input node and an output node. An inductor, one end of which is connected to the input node and the other end of which is connected the output node through the rectifier device, and a switch device connected between the other end and a low-potential-side line, and a first control circuit for the switch device are provided. The inductor stores energy while the switch device is on and releases the energy while the switch device is off. The rectifier device conducts such that the stored energy of the inductor is released.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inverter apparatus configured toconvert an input DC voltage into an AC voltage and output the ACvoltage.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 61-251480discloses an AC power supply apparatus provided with two invertercircuits. The AC power supply apparatus disclosed in Japanese UnexaminedPatent Application Publication No. 61-251480 produces a half-wave sinewave voltage by alternately driving the two inverter circuits, outputs apositive voltage from one of the inverter circuits, and outputs anegative voltage from the other of the inverter circuits, therebyoutputting an AC voltage. In other words, the AC power supply apparatusdisclosed in Japanese Unexamined Patent Application Publication No.61-251480 produces the positive half cycles and negative half cycles ofthe output AC voltage by using the two inverter circuits.

In a general inverter apparatus including the AC power supply apparatusdisclosed in Japanese Unexamined Patent Application Publication No.61-251480, a smoothing capacitor is provided in the input stage (on theinput side) of an inverter circuit. A current flowing through an AC loadvia the inverter circuits has a full-wave rectified waveform having afrequency of twice a utility power supply, whereby a ripple with thefrequency described above is generated in an input voltage input to theinverter circuits. When the input voltage varies, not only does itbecome difficult to control the inverter circuit, but also it becomesimpossible to transmit a sine wave current when the AC load is a powersystem (grid), thereby causing voltage waveform distortion. Thesmoothing capacitor is provided to suppress the ripple of an inputvoltage. The smaller the desired ripple of the input voltage aftersuppression, the higher the required capacitance of a capacitor, andhence, an electrolytic capacitor will be used, for example. However,since the lifetime of an electrolytic capacitor is short, the lifetimeof an apparatus is limited. Hence, instead of using a high-capacitancecapacitor, such as an electrolytic capacitor, it is desired to use acapacitor with a low capacitance, such as, for example, a film capacitorwith little change over the years as a smoothing capacitor. However,when a capacitor with a low capacitance, such as a film capacitor, isused as a smoothing capacitor, a sufficient ripple suppression effect isnot obtained.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide an inverterapparatus that sufficiently suppresses a ripple of an input voltage ofan inverter circuit without using a smoothing capacitor with a highcapacitance.

An inverter apparatus according to a preferred embodiment of the presentinvention includes an active filter circuit configured to step up andsmooth a DC voltage of an input power supply, and an inverter circuitconfigured to convert the DC voltage stepped up and smoothed by theactive filter circuit into an AC voltage. The active filter circuitincludes a buffer capacitor connected between an input node and anoutput node, a rectifier device, an inductor a first end of which isconnected to the input node and a second end of which is connected tothe output node through the rectifier device, a switch device connectedbetween the second end of the inductor and a low-potential-side line,and a switching control circuit for the switch device. The inductorstores energy while the switch device is on and releases the energywhile the switch device is off, and the rectifier device is configuredto conduct so as to allow the energy stored in the inductor to bereleased.

With this configuration, even during a period in which an AC voltageoutput from the inverter apparatus is near zero volts, energy is storedin the inductor by the switching of the switch device, and with theenergy as a voltage to charge the buffer capacitor, the capacitor ischarged to the DC voltage of the input power supply. During a period inwhich the AC voltage output from the inverter apparatus is near themaximum, the capacitor is discharged. As a result of this operation ofthe active filter, the ripple of the voltage input to the invertercircuit is reduced. Hence, the capacitance of a smoothing capacitorconnected to the input stage (input side) of the active filter circuitis reduced. As a result, a smoothing capacitor with a high capacitanceis not needed and, hence, for example, a film capacitor or the like withlittle change over the years can be used instead of an electrolyticcapacitor. Further, also during a period in which the current flowing inthe AC load is near zero, a current is input from the input powersupply, and during a period in which the current flowing through the ACload is near the maximum, a current is output from the buffer capacitorto the AC load through the inverter circuit, such that a DC power iseffectively drawn from the input power supply.

It is preferable that the switching control circuit is configured orprogrammed to control a voltage across the buffer capacitor, beingcharged, through PWM control of the switch device in such a manner thata voltage ripple of an output voltage of the active filter circuit issuppressed.

The voltage across the smoothing capacitor provided in the active filtercircuit varies in accordance with a current supplied from the activefilter circuit to the AC load through the inverter circuit. However, byswitching the switch device on/off through PWM control, variations inthe output voltage of the active filter circuit, that is, variations inthe voltage input to the inverter circuit are suppressed.

When the rectifier device is a diode, the circuit configuration issimplified since the switching control is not needed.

When the rectifier device is a MOSFET or an insulated gate bipolartransistor (IGBT), the conduction loss is reduced.

When the switch device is a MOSFET or an IGBT, the conduction loss isreduced. In particular, by including an IGBT, a high-speed operation ispossible and the breakdown resistance becomes large, such that highreliability is realized.

When the rectifier device and the switch device are portions of aplurality of power switch devices housed in an intelligent power module(IPM), the number of mounted components is decreased and the componentcost is reduced, such that reduction in size and cost is realized.

When the inverter circuit includes a bridge connection of four switchdevices and these switch devices are the power switches housed in theIPM, the number of the mounted components is further reduced and thecomponent cost is reduced, such that further reduction in size and costis realized.

An insulating DC-DC converter may be provided between the active filtercircuit and the input power supply. With this configuration, the inputpower supply and the inverter circuit is insulated from each other.

According to various preferred embodiments of the present invention,since the ripple of an input voltage input to an inverter circuit isreduced by the operation of an active filter, the capacitance of asmoothing capacitor connected to the input stage (input side) of theactive filter is reduced.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an inverter apparatus according to afirst preferred embodiment of the present invention.

FIG. 2 is a configuration diagram of a first control circuit.

FIG. 3 is a configuration diagram a second control circuit.

FIG. 4 is a waveform diagram of an output voltage of the inverterapparatus.

FIG. 5 is a diagram illustrating the gate signal waveforms of respectiveswitch devices when the phase angle of an output voltage is 0°.

FIG. 6 is a diagram illustrating the gate signal waveforms of therespective switch devices when the phase angle of an output voltage is45°.

FIG. 7 is a diagram illustrating the gate signal waveforms of therespective switch devices when the phase angle of an output voltage is90°.

FIG. 8 is a diagram illustrating the simulation conditions and thenumerical results of simulation.

FIG. 9A illustrates the current waveforms of the simulation results forcondition (1).

FIG. 9B illustrates the voltage waveforms of the simulation results forcondition (1).

FIG. 10A illustrates the current waveforms of the simulation results forcondition (2).

FIG. 10B illustrates the voltage waveforms of the simulation results forcondition (2).

FIG. 11A illustrates the current waveforms of the simulation results forcondition (3).

FIG. 11B illustrates the voltage waveforms of the simulation results forcondition (3).

FIG. 12 is a circuit diagram of an inverter apparatus according to asecond preferred embodiment of the present invention.

FIG. 13 is a circuit diagram of an inverter apparatus according to athird preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First PreferredEmbodiment

FIG. 1 is a circuit diagram of an inverter apparatus according to afirst preferred embodiment of the present invention. An inverterapparatus 1 according to the present preferred embodiment is preferablyused in, for example, a photovoltaic power generation system. Outputterminals P₀(+) and P₀(−) of the inverter apparatus 1 are connected to,for example, a power system load. The inverter apparatus 1 converts DCpower converted from solar energy into AC power and outputs the AC powerto an AC load. The inverter apparatus 1 of the present preferredembodiment preferably outputs a 200 V AC voltage with a frequency of 50Hz to the AC load, for example.

The inverter apparatus 1 includes an active filter circuit 10 and aninverter circuit 20. A DC power supply Vdc and a smoothing capacitor C1are connected to the input stage (input side) of the active filtercircuit 10. The DC power supply Vdc is, for example, a solar batterypanel. Hereinafter, an input voltage input from the DC power supply Vdcto the active filter circuit 10 will be denoted by Vin. The smoothingcapacitor C1 smoothes Vin.

The active filter circuit 10 includes a buffer capacitor Cdc(hereinafter, simply called a capacitor Cdc) connected between an inputnode and an output node and includes a rectifier device 11S. Here, theinput node is a high-potential-side connection node between the DC powersupply Vdc and the active filter circuit 10, and the output node is ahigh-potential-side connection node between the active filter circuitand the inverter circuit 20. The active filter circuit 10 also includesan inductor L1 a first end of which is connected to the input node andthe second end of which is connected to the output node through therectifier device 11S, a switch device 12S connected between the secondend of the inductor L1 and a ground line, and a first control circuit 30which is a control circuit for the switch device 12S.

The rectifier device 11S and the switch device 12S according to thepresent preferred embodiment are IGBTs and each include a body diode.The collector of the rectifier device 11S is connected to the outputside of the capacitor Cdc. The inductor L1 is connected to the inputside of the capacitor Cdc and the emitter of the rectifier device 11S.The collector of the switch device 12S is connected to the emitter ofthe rectifier device 11S and the emitter of the switch device 12S isconnected to the low-potential-side line of the active filter circuit10. The low-potential-side line is a line connected to thelow-potential-side of the DC power supply Vdc. The rectifier device 11Sand the switch device 12S are subjected to pulse-width-modulation (PWM)control performed by the first control circuit 30.

The rectifier device 11S and the switch device 12S are alternately(complementarily) switched on/off by the first control circuit 30. Whenthe switch device 12S is on and the rectifier device 11S is off, acurrent I₁₁ flows through a path going through the inductor L1 and theswitch device 12S. Energy is stored in the inductor L1 by the currentI₁₁. When the switch device 12S is switched off and the rectifier device11S is switched on, a current I₁₂ flows through a closed loop includingthe inductor L1, the rectifier device 11S, and the capacitor Cdc. Thecapacitor Cdc is charged by the current I₁₂. When the voltage across thecapacitor Cdc being charged is denoted by Vcdc and the output voltage ofthe active filter circuit 10 is denoted by Vout1, the output voltageVout1 is the sum of an input voltage Vin and the voltage Vcdc.

The inverter circuit 20 is connected to the output stage (output side)of the active filter circuit 10. The inverter circuit 20 generates thepositive half cycles and negative half cycles of an AC voltage from theoutput voltage Vout1 output from the active filter circuit 10, andoutputs these half cycles. In the inverter circuit 20, a series circuitincluding a switch device 21S and a switch device 22S and a seriescircuit including a switch device 23S and a switch device 24S areconnected in parallel with each other. These series circuits areconnected between the high-potential-side line and thelow-potential-side line in such a manner that the switch devices 21S and23S are on the high side and the switch devices 22S and 24S are on thelow side. Each of the switch devices 21S, 22S, 23S, and 24S is subjectedto PWM control performed by a second control circuit 40. Each of theswitch devices 21S, 22S, 23S, and 24S includes a body diode.

A connection node between the switch devices 21S and 22S is connected tothe output terminal P₀(+) through an inductor L2. The switch devices 23Sand 24S are connected to the output terminal P₀(−). An AC load isconnected to the output terminals P₀(+) and P₀(−), and an AC voltageoutput from the inverter apparatus 1 is applied to the AC load.

The inverter circuit 20, using the output voltage Vout1 output from theactive filter circuit 10 as a power supply voltage, outputs a positiveand negative AC voltage by switching the switch device 21S and theswitch device 24S on and off and switching the switch device 22S and theswitch device 23S on and off. Specifically, a current I₂₊ flows throughan AC load when the switch device 21S and the switch device 24S are onand the switch device 22S and the switch device 23S are off. A currentI²⁻ flows through the AC load when the switch device 22S and the switchdevice 23S are on and the switch device 21S and the switch device 24Sare off.

Hereinafter, the first control circuit 30 and the second control circuit40 will be described.

FIG. 2 is a configuration diagram of the first control circuit 30.Hereinafter, a current flowing through the inductor L1 is denoted byI_(L1). The first control circuit 30 is configured or programmed tosubject the rectifier device 11S and the switch device 12S to PWMcontrol at a frequency of 10 kHz, for example, in such a manner that thevoltage Vcdc across the capacitor Cdc, being charged, becomes aninstructed voltage (target voltage) Vcdc*.

A subtractor 31 is configured to compute an error between the voltageVcdc across the capacitor Cdc being charged and the instructed voltageVcdc*. A PI controller 32 is configured or programmed to compute aninstructed current I_(L1)* that is to flow through the inductor L1 byusing PI control (proportional and integral control) based on the errorcomputed by the subtractor 31. A subtractor 33 is configured to computean error between the target current I_(L1)* and the current I_(L1)flowing through the inductor L1. A PI controller 34 is configured orprogrammed to compute an instructed voltage V_(L1)* to be applied to theinductor L1 by using PI control (proportional and integral control)based on the error computed by the subtractor 33. A comparator 35 isconfigured to compare the result obtained by the PI controller 34 with atriangle wave with a frequency of 10 kHz, thus outputting a PWM wave.The PWM wave output by the comparator 35 is input to the rectifierdevice 11S. The PWM wave output by the comparator 35 is inverted by aninverter 36 and the inverted PWM wave is input to the switch device 12S.

FIG. 3 is a configuration diagram of the second control circuit 40. Thesecond control circuit 40 is configured to subject the switch devices21S, 22S, 23S, and 24S to PWM control at a frequency of 5 kHz, forexample, in such a manner that an output current Iout2 of the invertercircuit 20 becomes an instructed current (target current) Iout2*.

A multiplier 41 is configured to multiply the instructed current Iout2*by a frequency (50 Hz of an AC load in the present preferred embodiment)to be set, and outputs the result to a subtractor 42. The subtractor 42is configured to compute an error between the output of the multiplier41 and the output current Iout2 output from the inverter circuit 20, andoutput the error to a PI controller 43. The PI controller 43 isconfigured to obtain an instructed current I_(L2)* by using PI controlbased on this error. The instructed current I_(L2)* is a current to bemade to flow through the inductor L2.

A comparator 44 is configured to output a PWM wave to generate thepositive half cycles of an AC voltage. The comparator 44 is configuredto compare the output of the PI controller 43 with a triangle wave witha frequency of 5 kHz, for example, thus generating a PWM wave. The PWMwave output by the comparator 44 is input to the switch device 21S. ThePWM wave output by the comparator 44 is inverted by an inverter 45 andthe inverted PWM wave is input to the switch device 24S.

A comparator 47 is configured to output a PWM wave. A multiplier 46 isconfigured to multiply the output of the PI controller 43 by −1 andoutput the result to the comparator 47. Hence, the output signal of themultiplier 46 is input to the comparator 47. The comparator 47 isconfigured to compare the output signal of the multiplier 46 with atriangle wave with a frequency of 5 KHz, for example, thus generating aPWM wave. The PWM wave output by the comparator 47 is input to theswitch device 23S. The PWM wave output by the comparator 47 is invertedby an inverter 48 and the inverted PWM wave is input to the switchdevice 22S.

Hereinafter, a gate signal output by the first control circuit 30 to therectifier device 11S and the switch device 12S, and a gate signal outputby the second control circuit 40 to the switch devices 21S, 22S, 23S,and 24S will be described.

FIG. 4 is a waveform diagram of an output voltage Vout2 of the inverterapparatus 1. In the present preferred embodiment, the first controlcircuit 30 and the second control circuit 40 perform PWM control withdifferent duty ratios in accordance with a phase angle φ of the outputvoltage Vout2. Hereinafter, the PWM control at the time when the phaseangle φ of the output voltage Vout2 preferably is near 0°, 45°, and 90°,for example, as illustrated in FIG. 4 will be described.

FIG. 5 is a diagram illustrating the gate signal waveforms of therespective switch devices when the phase angle φ of the output voltageVout2 is near 0°. FIG. 6 is a diagram illustrating the gate signalwaveforms of the respective switch devices when the phase angle φ of theoutput voltage Vout2 is near 45°. FIG. 7 is a diagram illustrating thegate signal waveforms of the respective switch devices when the phaseangle φ of the output voltage Vout2 is near 90°. In each of FIG. 5, FIG.6, and FIG. 7, the vertical axis represents applied voltage and thehorizontal axis represents time.

As described above, the gate signal (PWM wave) output by the firstcontrol circuit 30 to the rectifier device 11S and the switch device 12Shas a duty ratio which changes in accordance with the voltage Vcdc* andthe input voltage Vin. In the waveforms illustrated in FIG. 5, FIG. 6,and FIG. 7, a setting is made in such a manner that the amplitude (peakto peak) of the instructed voltage Vcdc* becomes about 95.5 V, forexample. In this simulation, the degree of the ripple of an inputcurrent Iin is determined while the input voltage Vin is kept constant.In fact, Vin changes in accordance with the internal resistance of aninput power supply, the capacitance of the smoothing capacitor C1, andthe input current Iin. However, since the ripple of the input currentIin is suppressed, the effect of the suppression of a change in theinput voltage Vin is capable of being determined. Since the inputvoltage Vin is made to be constant as described above in thissimulation, the amount of change in the duty ratio in accordance withthe instructed voltage Vcdc* is small and the duty ratio is nearlyconstant. Hence, gate signal voltage waveforms applied to the rectifierdevices 11S and the switch devices 12S illustrated in FIG. 5, FIG. 6,and FIG. 7 are also nearly the same.

When the phase angle φ is nearly 0°, the on-duty ratio of the switchdevice 21S and the switch device 23S is the same or approximately thesame as the on-duty ratio of the switch device 22S and the switch device24S. As the phase angle φ is increased to 45° and further to 90°, theon-duty ratio of the switch device 21S and the switch device 23S of theinverter circuit 20 becomes larger than the on-duty ratio of the switchdevice 22S and the switch device 24S.

Next, the current waveform and the voltage waveform of the inverterapparatus 1 configured as above, in which the switch devices aresubjected to PWM control, will be described. Hereinafter, the result ofa simulation performed in the inverter apparatus 1 will be illustrated.Regarding the simulation conditions, preferably the output voltage Vout1of the active filter circuit 10 is 400 V on average, and the outputvoltage Vout2 of the inverter circuit 20 is an AC voltage of 240 V witha frequency of 50 Hz, for example. In FIG. 1, it is assumed that thecapacitance C1 is 100 μF, the capacitance Cdc is 50 μF, the inductanceL1 is 6 mH, and the inductance L2 is 36 mH, for example. Each of therectifier device 11S and the switch device 12S preferably is subjectedto PWM control at a frequency of 10 kHz, for example, and each of theswitch devices 21S, 22S, 23S, and 24S preferably is subjected to PWMcontrol at a frequency of 5 kHz, for example.

FIG. 8 is a diagram illustrating the simulation conditions and thenumerical results of the simulation. In FIG. 8, the simulationconditions and numerical results for the respective cases of condition(1), condition (2), and condition (3) are illustrated. FIG. 9Aillustrates the current waveforms of the simulation results forcondition (1), and FIG. 9B illustrates the voltage waveforms of thesimulation results for condition (1). FIG. 10A illustrates the currentwaveforms of the simulation results for condition (2), and FIG. 10Billustrates the voltage waveforms of the simulation results forcondition (2). FIG. 11A illustrates the current waveforms of thesimulation results for condition (3), and FIG. 11B illustrates thevoltage waveforms of the simulation results for condition (3).

Referring to FIG. 8, as the simulation conditions, the input voltageVin, an average Vcdcave for the voltage Vcdc across the capacitor Cdcbeing charged, the amplitude (peak to peak) ΔVcdc of the voltage Vcdcacross the capacitor Cdc, and the output power Pout of the active filtercircuit 10 are provided. As the simulation results, Icdc is a currentflowing through the capacitor Cdc, and I_(L1) is a current flowingthrough the inductor L1. The horizontal axis of each of the graphsillustrated in FIG. 9, FIG. 10, and FIG. 11 represents time in seconds.

Under condition (1), the input voltage Vin of 150 V is input to theactive filter circuit 10. In this case, the input current Iin input tothe active filter circuit 10 is a current having an average of about 2.0A, for example, and including a ripple. As a result of the rectifierdevice 11S and the switch device 12S being subjected to on/off control,the current Icdc, the current I_(L1), and an output current Iout1 becomecurrents each including a ripple. The current Icdc is a current with amaximum value of about 1.26 A, for example, (refer to FIG. 8), and flowsas a current including a ripple as a result of the switching of theswitch device 12S. The current I_(L1) preferably has a maximum value ofabout 4.1 A, for example, (refer to FIG. 8), and always flows in thesame direction. The current Iout1 preferably has a maximum value ofabout 1.26 A, for example, similarly to the current Icdc. An AC currentincluding positive half cycles and negative half cycles is generated bythe switch devices 21S, 22S, 23S, and 24S subjected to on/off controlfrom the output of the active filter circuit 10, such that the currentIout2 is output from the inverter circuit 20.

Further, under condition (1), a voltage of from about −150 V to about200 V, for example, is applied to the inductor L1. The voltage Vcdc ofabout 250 V, for example, is applied to the capacitor Cdc. Specifically,the voltage Vcdc preferably has an AC waveform with the center at about250 V and the amplitude of about 76.4 V, for example, (refer to FIG. 8).The output voltage Vout1 of the active filter circuit 10 becomes avoltage which is the sum of the voltage Vin and the voltage Vcdc. Inother words, the voltage Vout1 preferably becomes approximately 400 V,for example. An AC voltage including positive half cycles and negativehalf cycles is generated from the output of the active filter circuit 10by the switch devices 21S, 22S, 23S, and 24S subjected to on/offcontrol, such that the voltage Vout2 is output from the inverter circuit20.

Under condition (2), the input voltage Vin of about 200 V, for example,preferably is input to the active filter circuit 10. In this case, theinput current Iin input to the active filter circuit 10 is a currentincluding a ripple with an average of about 1.5 A, for example. As aresult of the rectifier device 11S and the switch device 12S beingsubjected to on/off control, the current Icdc, the current I_(L1), andthe output current Iout1 become currents each including a ripple. Thecurrent Icdc is a current with a maximum value of about 1.5 A, forexample, (refer to FIG. 8), and flows as a current including a ripple asa result of the switching of the switch device 12S. The current I_(L1)preferably has a maximum value of about 4.0 A, for example, (refer toFIG. 8), and always flows in the same direction. The current Iout1preferably has a maximum value of about 1.5 A, for example, similarly tothe current Icdc. An AC current including positive half cycles andnegative half cycles is generated by the switch devices 21S, 22S, 23S,and 24S subjected to on/off control from the output of the active filtercircuit 10, such that the current Iout2 is output from the invertercircuit 20.

Further, under condition (2), a voltage of from about −200 V to about200 V, for example, preferably is applied to the inductor L1. Thevoltage Vcdc of about 200 V, for example, preferably is applied to thecapacitor Cdc. Specifically, the voltage Vcdc preferably has an ACwaveform with the center at about 200 V and the amplitude of about 95.5V, for example, (refer to FIG. 8). The output voltage Vout1 of theactive filter circuit 10 becomes a voltage which is the sum of thevoltage Vin and the voltage Vcdc. In other words, the voltage Vout1preferably becomes approximately 400 V, for example. An AC voltageincluding positive half cycles and negative half cycles is generatedfrom the output of the active filter circuit 10 by the switch devices21S, 22S, 23S, and 24S subjected to on/off control, such that thevoltage Vout2 is output from the inverter circuit 20.

Under condition (3), the input voltage Vin of about 250 V, for example,preferably is input to the active filter circuit 10. In this case, theinput current Iin input to the active filter circuit 10 preferably is acurrent including a ripple with an average of about 1.25 A, for example.As a result of the rectifier device 11S and the switch device 12S beingsubjected to on/off control, the current Icdc, the current I_(L1), andthe output current Iout1 become currents each including a ripple. Thecurrent Icdc preferably is a current with a maximum value of about 1.6A, for example, (refer to FIG. 8), and flows as a current including aripple as a result of the switching of the switch device 12S. Thecurrent I_(L1) preferably has a maximum value of about 3.7 A, forexample, (refer to FIG. 8), and always flows in the same direction. Thecurrent Iout1 preferably has a maximum value of about 1.6 A, forexample, similarly to the current Icdc. An AC current including positivehalf cycles and negative half cycles is generated by the switch devices21S, 22S, 23S, and 24S subjected to on/off control from the output ofthe active filter circuit 10, such that the current Iout2 is output fromthe inverter circuit 20.

Further, under condition (3), a voltage of from about −250 V to about200 V, for example, preferably is applied to the inductor L1. Thevoltage Vcdc of about 150 V, for example, preferably is applied to thecapacitor Cdc. Specifically, the voltage Vcdc preferably has an ACwaveform with the center at about 150 V and the amplitude of about127.32 V, for example (refer to FIG. 8). The output voltage Vout1 of theactive filter circuit 10 becomes a voltage which is the sum of thevoltage Vin and the voltage Vcdc. In other words, the voltage Vout1preferably becomes approximately 400 V, for example. An AC voltageincluding positive half cycles and negative half cycles is generatedfrom the output of the active filter circuit 10 by the switch devices21S, 22S, 23S, and 24S subjected to on/off control, such that thevoltage Vout2 is output from the inverter circuit 20.

As can be seen from the waveforms of the simulation results, byappropriately controlling the voltage across the capacitor Cdc of theactive filter circuit 10, the ripple of the input current Iin isdecreased. This is due to energy movement (a buffering operation) in theinductor L1 and the capacitor Cdc of the active filter circuit 10.Hence, there is no need to increase the capacitance of the smoothingcapacitor C1 to reduce the ripple of a DC current input to the activefilter circuit 10. Further, since Iin flows even when Iout1 has a phaseangle near zero, power is effectively drawn from the DC power supplyVdc. In other words, in the case where the DC power supply Vdc is asolar battery panel, DC power is effectively drawn from solar energy.

Note that in the present preferred embodiment, an intelligent powermodule (IPM) which includes six IGBTs as a single module, for example,preferably is used. In other words, a configuration preferably is usedin which, among the six IGBTs, four of them are the switch devices 21S,22S, 23S, and 24S, and the remaining two are the rectifier device 11Sand the switch device 12S, for example.

Second Preferred Embodiment

Hereinafter, a second preferred embodiment of the present invention willbe described. Unlike the first preferred embodiment, the secondpreferred embodiment has a configuration in which the rectifier device11S according to the first preferred embodiment includes a diode and theswitch devices include MOSFETs.

FIG. 12 is a circuit diagram of an inverter apparatus according to thesecond preferred embodiment. An inverter apparatus 1A according to thesecond preferred embodiment includes an active filter circuit 11 and aninverter circuit 21. The active filter circuit 11 includes a capacitorCdc, an inductor L1, a diode (rectifier device) D1, and a switch device3S. The capacitor Cdc is connected in series with a high-potential-sideline of the active filter circuit 10. The cathode of the diode D1 isconnected to the output side of the diode D1. The inductor L1 isconnected between the input side of the capacitor Cdc and the anode ofthe diode D1. The drain of the switch device 3S is connected to theanode of the diode D1, and the source of the switch device 3S isconnected to a low-potential-side line of the active filter circuit 10.The switch device 3S is subjected to PWM control performed by a firstcontrol circuit 30A.

The on/off control of the switch device 3S is similar to that for theswitch device 12S according to the first preferred embodiment. Thecapacitor Cdc is charged as a result of the switch device 3S beingsubjected to PWM control performed by the first control circuit 30A.When the switch device 3S is on, a current I₁₁ flows through a pathgoing through the inductor L1 and the switch device 3S. Energy is storedin the inductor L1 by the current I₁₁. When the switch device 3S isswitched off, a current I₁₂ flows through a closed loop including theinductor L1, the diode D1, and the capacitor Cdc. A current output fromthe inductor L1 in which the electric energy has been stored is added tothe current I₁₂. The capacitor Cdc is charged by the current I₁₂.

In the inverter circuit 21, a switch device 41S and a switch device 42Sconnected in series with each other are connected in parallel with aswitch device 43S and a switch device 44S connected in series with eachother. In more detail, the drain of the switch device 41S is connectedto a high-potential-side line of the inverter circuit 20, and the sourceof the switch device 41S is connected to the drain of the switch device42S. The source of the switch device 42S is connected to alow-potential-side line of the inverter circuit 20. The drain of theswitch device 43S is connected to the high-potential-side line of theinverter circuit 20, and the source of the switch device 43S isconnected to the drain of the switch device 44S. The source of theswitch device 44S is connected to the low-potential-side line of theinverter circuit 20. Each of the switch device 41S, the switch device42S, the switch device 43S, and the switch device 44S is subjected toPWM control performed by a second control circuit 40A. The on/offcontrol of the switch devices 41S, 42S, 43S, and 44S is similar to thatof the switch devices 21S, 22S, 23S, and 24S according to the firstpreferred embodiment.

In this manner, also when MOSFETs are used as the switch devices,effects similar to those in the first preferred embodiment are obtained.Further, by using the diode D1 as a rectifier device, the switchingcontrol for the device is not required, resulting in a simplifiedcircuit configuration.

Note that in the present preferred embodiment, a MOSFET may be usedinstead of the diode D1. In this case, an intelligent power module (IPM)which is configured as a single module including six MOSFET devices maybe used. In other words, a configuration may be used in which four ofthe six MOSFETs are switch devices of an inverter circuit and theremaining two are a rectifier device and a switch device for the activefilter circuit, for example.

Third Preferred Embodiment

Hereinafter, a third preferred embodiment of the present invention willbe described. An inverter apparatus according to the present preferredembodiment has a configuration in which an insulating DC-DC converter isprovided between the input stage (input side) of the smoothing capacitorC1 of the second preferred embodiment and the DC power supply Vdc.

FIG. 13 is a circuit diagram of an inverter apparatus according to thethird preferred embodiment. The active filter circuit 11 and theinverter circuit 21 provided in an inverter apparatus 1B preferably arethe same as those in the second preferred embodiment. Note that theinverter apparatus 1B may have a configuration including the activefilter circuit 10 and the inverter circuit 20 according to the firstpreferred embodiment.

On the primary side of an insulating DC-DC converter 13, a full-bridgecircuit is provided and includes switching devices 51S, 52S, 53S, and54S made of MOSFETs. A control circuit is connected to the gates of theswitching devices 51S, 52S, 53S, and 54S to perform PWM control.

A primary winding np of an insulating transformer T is connected to theoutput of the full-bridge circuit through a capacitor C3. The capacitorC3 and the primary winding np define a resonant circuit. A diode bridgerectifier circuit including diodes D11, D12, D13, and D14 is connectedto the secondary side of the insulating transformer T. In this manner, aresonant full-bridge converter is provided. The inductor L3, thesmoothing capacitor C1, and the active filter circuit 11 are connectedto the output stage of the insulating DC-DC converter 13.

In the inverter apparatus 1B, as a result of a state in which one of thecombination of the switching devices 51S and 54S and the combination ofthe switching devices 52S and 53S is on at the same time and the otherof the combinations is off at the same time being repeated, a resonantcurrent generated by a resonant circuit including the capacitor C3 andthe like flows through the primary winding np of the insulatingtransformer T. When the current flows through the primary winding np ofthe insulating transformer T, an electromotive force is generated in asecondary winding ns of the insulating transformer T, such that power istransmitted to the secondary side of the insulating transformer T. Thepower transmitted to the secondary side is rectified by the diode bridgerectifier circuit and is output to the smoothing capacitor C1. Theoperations of the active filter circuit 11 and the inverter circuit 21are the same as those of the first and second preferred embodiments.

In the present preferred embodiment, as a result of the output voltageof the DC power supply Vdc being input to the smoothing capacitor C1through the insulating DC-DC converter 13, stable supply of power to theactive filter circuit 11 becomes possible, compared with the case inwhich the DC power supply Vdc is directly connected to the smoothingcapacitor C1.

By making the insulating DC-DC converter (resonant full-bridgeconverter) 13 operate without control, that is, by driving the switchdevices at a duty ratio of approximately 50% with a dead time betweenthe on periods, maximum power point tracking control (MPPT control) isrealized. In other words, when the input power supply is a solar batterypanel, in order to draw maximum power from the solar battery, it isnecessary to control a voltage so as to make the product of a currentand the voltage become maximum. The current-voltage (I-V)characteristics may change in accordance with solar irradiance or themodule temperature and, hence, it is important to automatically followthe optimal voltage to obtain the maximum power. Hence, the maximumpower is drawn from the solar battery as a result of the insulatingDC-DC converter 13 performing maximum power point tracking control (MPPTcontrol).

Note that the design of the specific configurations of the inverterapparatuses according to the preferred embodiments described above maybe appropriately changed. In the case of multiple levels, variousmethods such as an intermediate voltage clamp method, a flying capacitormethod, and a cascade method may be used, for example. The operationsand effects described in the above preferred embodiments have beenprovided only as most preferable operations and effects realized by thepresent invention, and the operations and effects of the presentinvention are not limited by those described in the above preferredembodiments.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. An inverter apparatus comprising: an activefilter circuit configured to step up and smooth a DC voltage of an inputpower supply; and an inverter circuit configured to convert the DCvoltage stepped up and smoothed by the active filter circuit into an ACvoltage; wherein the active filter circuit includes: a buffer capacitorconnected between an input node and an output node; a rectifier device;an inductor a first end of which is connected to the input node and asecond end of which is connected to the output node through therectifier device; a switch device connected between the second end ofthe inductor and a low-potential-side line; and a switching controlcircuit for the switch device; wherein the inductor is configured tostore energy while the switch device is on and release the energy whilethe switch device is off, and the rectifier device is configured toconduct so as to allow the energy stored in the inductor to be released.2. The inverter apparatus according to claim 1, wherein the switchingcontrol circuit is configured or programmed to control a voltage acrossthe buffer capacitor, being charged, through PWM control of the switchdevice such that a voltage ripple of an output voltage of the activefilter circuit is suppressed.
 3. The inverter apparatus according toclaim 1, wherein the rectifier device is a diode.
 4. The inverterapparatus according to claim 1, wherein the rectifier device is a MOSFETor an insulated gate bipolar transistor, and the switching controlcircuit is configured to complementarily drive the rectifier device andthe switch device.
 5. The inverter apparatus according to claim 1,wherein the switch device is a MOSFET or an insulated gate bipolartransistor.
 6. The inverter apparatus according to claim 1, wherein therectifier device and the switch device are portions of a plurality ofpower switch devices housed in an intelligent power module.
 7. Theinverter apparatus according to claim 6, wherein the inverter circuitincludes a bridge connection of four switch devices and the four switchdevices are the power switch devices housed in the intelligent powermodule.
 8. The inverter apparatus according to claim 1, furthercomprising an insulating DC-DC converter connected between the activefilter circuit and the input power supply.
 9. The inverter apparatusaccording to claim 1, wherein when the switch device is switched off andthe rectifier device is switched on, a current flows through a closedloop including the inductor, the rectifier device, and the capacitor.10. The inverter apparatus according to claim 1, wherein the inverterdevice is configured to generate positive half cycles and negative halfcycles of an AC voltage and output the positive half cycles and negativehalf cycles.
 11. The inverter apparatus according to claim 1, whereinthe inverter circuit includes a first series circuit including first andsecond switch devices and a second series circuit including third andfourth switch devices, connected in parallel with each other.
 12. Theinverter apparatus according to claim 11, further comprising a controlcircuit configured to perform PWM control of the first, second, thirdand fourth switch devices.
 13. The inverter apparatus according to claim12, wherein the switching control circuit and the control circuit areconfigured to perform PWM control with different duty ratios inaccordance with a phase angle of an output voltage.
 14. The inverterapparatus according to claim 6, wherein the power switch devices housedin the intelligent power module include six insulated gate bipolartransistors.
 15. The inverter apparatus according to claim 14, whereinfour of the six insulated gate bipolar transistors define switchdevices, a fifth of the six insulated gate bipolar transistors definesthe rectifier device, and a sixth of the six insulated gate bipolartransistors defines the switch device connected between the second endof the inductor and a low-potential-side line.
 16. The inverterapparatus according to claim 1, wherein the rectifier device includes adiode and switch devices defined by MOSFETs.
 17. The inverter apparatusaccording to claim 1, further comprising a smoothing capacitor and aninsulating DC-DC converter between an input of the smoothing capacitorand the input power supply.
 18. The inverter apparatus according toclaim 17, further comprising a full bridge circuit provided on a primaryside of the insulating DC-DC converter.
 19. The inverter apparatusaccording to claim 18, further comprising a resonant circuit including acapacitor and a primary winding of an insulating transformer connectedto the full bridge circuit.
 20. A power generation system comprising theinverter apparatus according to claim 1.